This application is based on application No. 10-354959 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal composition, and more particularly to a liquid crystal composition which exhibits a cholesteric phase in a room temperature and selectively reflects light of a specified wavelength in the visible spectrum. The present invention also relates to a non-volatile liquid crystal light modulating device which uses such a liquid crystal composition.
2. Description of Related Art
Recently, studies have been made to develop reflective liquid crystal displays by using chiral nematic liquid crystal which is produced by adding a chiral agent to nematic liquid crystal to cause the liquid crystal to exhibit a cholesteric phase in a room temperature. Such a liquid crystal display makes a display by switching between a planar state and a focal-conic state in accordance with the level of the pulse voltage applied thereto.
At present, however, in such a reflective liquid crystal display, the contrast between the planar state and the focal-conic state is not sufficiently high, and liquid crystal displays of this type which are satisfactory in properties, such as reflectance, phase transition temperature, etc., are yet to be developed.
It is a primary object of the present invention to provide an improved liquid crystal composition and an improved liquid crystal light modulating device. Further to this, it is other object of the present invention to provide a liquid crystal composition and a liquid crystal light modulating device which are good in properties, such as reflectance, phase transition temperature, etc., and have high contrast.
In order to attain the object, in a liquid crystal composition which contains liquid crystal and a plurality of chiral agents and exhibits a cholesteric phase in a room temperature, according to the present invention, the chiral agents include two chiral agents which have mutually opposite helical senses (cause liquid crystal to twist in mutually opposite directions), and the total content of the chiral agents in the liquid crystal composition is 10 to 45 wt %. Also, a liquid crystal light modulating device according to the present invention has such a liquid crystal composition between a pair of substrates, at least one of which is transparent.
According to the present invention, a plurality of chiral agents, including two chiral agents which have mutually opposite helical senses, are used. A chiral agent causes liquid crystal to twist and to exhibit a cholesteric phase in a room temperature, and the wavelength of light to be selectively reflected by the liquid crystal depends on the content of the chiral agent. By adding two chiral agents which have mutually opposite helical senses, domains which are different in twist direction are caused in the liquid crystal mixture. Thereby, in the planer state, light capable of going out to the observer side becomes large, which means an improvement in Y value (luminous reflectance) and reflectance, thereby resulting in an improvement in contrast.
It is thought that the improvement is achieved inasmuch as 1) both elliptical polarized light components of clockwise and counterclockwise directions are used for the reflection, and 2) some light components are scattered due to the unevenness among domains.
The total content of the chiral agents in the liquid crystal composition, in consideration for the reliability of the liquid crystal composition, shall be at most 45 wt %. Using a liquid crystalline compound as the principle chiral agent suppresses separation of the mixed chiral agents and crystallization of the chiral agents and improves the non-volatility. Also, it is possible to improve in phase transition temperature by using a chiral agent which is not so temperature dependent in a large amount. By use of the characteristics of various chiral agents, a liquid crystal composition with desired properties can be obtained.